upload-pack: use 'struct upload_pack_data' in upload_pack()

upload-pack: use 'struct upload_pack_data' in upload_pack()

As we cleanup 'upload-pack.c' by using 'struct upload_pack_data'
more thoroughly, let's use 'struct upload_pack_data' in
upload_pack().

This will make it possible in followup commits to remove a lot
of static variables and local variables that have the same name
and purpose as fields in 'struct upload_pack_data'. This will
also make upload_pack() work in a more similar way as
upload_pack_v2().

Signed-off-by: Christian Couder <redacted>
Signed-off-by: Junio C Hamano <redacted>